In Gustav Gloom and the Four Terrors, things are not always what they seem in the Gloom Mansion. Gustav decides that he needs to rescue his father from the Dark Country, but he can't do that without Fernie's help. After Gustav convinces Fernie's safety-conscious father to enter the Gloom Mansion with Fernie and Pearlie, there is a prison break from the Hall of Shadow Criminals and it's up to Fernie to save her family and Gustav before it's too late.